1. Technical Field
The present invention relates to stampers applied in the manufacture of optical disks representative of which are DVDs and CDs.
2. Description of the Related Art
Optical disks such as DVDs and CDs are formed with a stamper in a mold made of a metal such as nickel, by pouring molten polycarbonate into the mold, and on the side having the pit-formed surface after molding, forming a vapor-deposition layer and a protective layer to yield an optical disk. By a conventional manufacturing technique of this sort, in parting the post-molded disk from the stamper, articles that mold-releasing irregularities cause to be defective a liable to be produced, with the defective rate amounting to 10-20 percent of the total. Specifically, although the disk is supposed to separate from the mold after it is opened, areas that then release easily from the mold and areas that do not will arise, and in the areas that release reluctantly the bumps furnished on the stamper side of the mold strike the edges on the opening rims of the pits transferred onto the disk, which ends up deforming the pits' contour. This mold-release defect, which spreads mottled-like on the impressed face of the disk, is referred to as clouding due to its nebulous look; and disks defective in outward appearance are not shipped out but are disposed of.
In connection with this kind of optical-disk manufacturing method, a proposal for improving the releasability of disks from mold stampers has been made known to the public in Japanese Pat. Pub. No. 2519985. Therein, after the stamper reverse face that contacts the inner surface of the die has been polished to under 0.1 s, by grinding the just-polished face with a polishing material such as sandpaper to roughen the surface texture to 0.3 to 0.2 s, the surface area of the stamper with respect to the die is decreased to suppress heat transmission to the die. Nevertheless, roughening the reverse face of the stamper unavoidably leads to the occurrence between the stamper and the die of relative displacement following on differences in thermal expansion and shrinkage rates, which makes the stamper reverse face quickly wear out and shortens its life span.
As a result of investigating causes giving rise to mold-release defects, the present inventor noted irregularities in the manner in which heat is conducted from the stamper to the die and came upon the scheme for the present invention. The temperature of the molten resin poured into the mold and the die temperature each differ somewhat during molding according to the molding conditions and the type of disk that is to be molded, but for the most part molten-resin temperatures are generally 300 to 400° C.; die temperatures 80 to 120° C. In this connection, stampers are made with nickel plate as a base material, and nickel's thermal diffusivity (coefficient of thermal diffusion) per unit time is fairly large (0.085 m2/h). Consequently, it is conceivable that simultaneously with the molten resin that is filed into the die coming into contact with the molding surface of the stamper, the heat of the molten resin gets absorbed by the die via the stamper, rapidly cooling and hardening the molten-resin surface in contact with the stamper. As long as this hardening occurs equally over the entire disk including the land surface, and further as long as shrinkage of the resin takes place uniformly until the mold is opened, there will be no room for mold-release defects to arise; but in practice a large volume of defective products are produced, as noted earlier.
The molten resin that, contacting the stamper, locally is rapidly cooled and hardened as noted above divides into a hardened skin layer and a flowing layer that flows into the central space within the mold cavity; but the facility with which the skin layer traces the external configuration of the bumps deteriorates remarkably once hardening has set in. Consequently, the configurations of the bumps on the stamper do not get transferred accurately. In order to avert this sort of transfer defect and furthermore, reduce the flow resistance of the molten resin within the mold cavity, an injection-compression molding technique has been adopted. Therein, the force with which the mold is clamped in filling the molten resin into the cavity is lowered; then raising the mold-clamping force simultaneously with terminating the molten-resin fill causes the molten resin to adhere to the bumps, improving the transferability. Nevertheless, although the injection-compression molding technique heightens the molten resin transferability insofar as the mold clamping force is increased, the technique does not go so far as to nullify the local rapid cooling and hardening of the molten resin surface; presumably, because local internal stress is produced due to hardening inconsistencies as noted earlier, areas that release easily from the mold and areas that do not are created and the bumps furnished on the stamper side deform the configurations of the pits, which ends up clouding the disk.